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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2021)
The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.
Computational methods were employed to study the surprising 2004 synthesis of de-camethyldizincocene, Zn2(η5−C5Me5)2, which was the first molecule to have a di- rect, unbridged bond between two first-row transition metals. The computational re- sults show that the methyl groups of decamethylzincocene, Zn(η5−C5Me5)(η1−C5Me5), affect the transition-state stability of its reaction with ZnEt2 (or ZnPh2) through steric hindrance, and this allows a counter-reaction, the homolytic dissociation of Zn(η5−C5Me5)(η1−C5Me5) into Zn(η5−C5Me5)• and (η1−C5Me5)• radicals to occur, and since no such steric hindrance exists when zincocene, Zn(η5−C5H5)(η1−C5H5), is used as a reactant, its dissociation never occurs.Experimentally, it was found that forming decamethyldizincocene is more efficient when using a reducing agent (e.g., KH) and ZnCl2 as opposed to a ZnR2 reagent. The computational results show that the methyl groups of decamethylzincocene have a similar indirect effect on the reaction. When zincocene is used, the reaction with KH favours the route that results in the formation of the zincate, K+[Zn(η1−C5H5)3]−. However, the path of formation for the zincate K+[Zn(η1−C5Me5)3]− is simply not favourable kinetically or hermodynamically, so the formation of decamethyldizincocene is the only option when Zn(η5−C5Me5)(η1−C5Me5)is used.Finally, it had been found that a particular chiral neutral zirconium amidate com- plex makes an effective catalyst for cyclizing primary aminoalkenes in a highly enan- tioselective fashion. The computational analysis indicates that the reason why one enantiomer is favoured is because of steric interference with the catalytic backbone that is non-existent with the other enantiomer, and this affects the major transition states throughout the cycle. This finding agrees with the experimental hypothesis.
The ﬁrst part of the thesis examines, using density functional theory (DFT) calculations, the effectsof introducing transition metals (TMs) into different systems, including small Au clusters; carbon nanotubes (CNTs); pristine and defected boron nitride nanotubes (BNNTs). The results show that the frontier molecular orbitals of the TM modiﬁed systems are usually localized around the doping site and the reactivities of these systems are often improved. In the case of small TM clusters, both PtAum and Aun tend to be planar in their ground state. N₂ and O₂ adsorption onto these clusters results in different adsorption conﬁgurations due to different orbital interactions. With regard to the TM modiﬁed CNTs, the endo-TM-doped CNTs are less stable than the corresponding exo-doped isomers due to the large geometric strain caused by deformation. The exo-doped SWCNTs are better electron donors than their endo-doped counterparts. As for the Pt modiﬁed BNNTs, binding energy analysis revealed that a Pt atom can move freely on a pristine BNNT. But the Pt atom is trapped between the B B bond at the defect site if a Stone-Wales defect exists. In both cases, the hosting BNNTs are wide-gap semiconductors with slightly improved reactivities. In comparison, BNNTs doped with Pt atoms are narrow-gap semiconductors with greatly enhanced reactivities.Both MP2 EPR-III and B3LYP EPR-III calculations were used to optimize butyl isomers and calculate hyperﬁne coupling constants (HFCCs) to explain experimental data. The C-Mu distance was elongated to 1.076 times the corresponding equilibrium C-H bond length to mimic vibrationally average β-muoniated radical geometries so as to calculate the muon HFCCs. Some muon HFCCs and most proton HFCCs were satisfactorily reproduced by the B3LYP calculations due to error cancellations, whereas other cases were better predicted by the MP2 calculations. The torsional potential energy surface (PES) of the sec-butyl radical was also studied. The cis conformation, which was observed in experiments, was unobtainable using some common DFT functionals, but can be identiﬁed by calculations using wavefunction theory or some modiﬁed hybrid functionals. Changes in basis set only modify the shape of the PES slightly.
My Ph. D. work is about theoretical basis and applications of density functional theory (DFT). DFT has demonstrated a good balance between computing costand accuracy, so it has become one of the most popular daily-used quantum chemistry methods.The first part of my work is about the asymptotic behavior of finite-system wave-functions. The exponential decaying asymptotic behavior is confirmed andthe structure of the prefactors is further explored. By comparing the asymptotic behavior of the Dyson orbitals and the Kohn-Sham orbitals, we have also provided a physical interpretation of the Kohn-Sham orbital energies.Then we want to rebut the theory of "unconventional density variation" proposed more than 20 years ago. Supported by theoretical analysis and numericalevidence, we proved that all density variations are the same in nature.We have also extended two total energy functionals suggested before to the Hartree-Fock method. Numerical tests on different molecules show these functionals are very promising in accelerating the SCF convergence of quantum chemistry calculations.Finally, we completed a comprehensive theoretical study on the tautomers of pyridinethiones. Many molecular properties predicted from theory are comparedwith those got from experiments. The dominant forms of the tautomers are confirmed to be the thione forms. This demonstrates the power of DFT methods, and this work can serve as a reference for studying similar molecules.
Master's Student Supervision (2010 - 2020)
Carbon nanotubes (CNTs) are of great significance due to their almost unlimited potential applications. Transition metal doped CNTs in particular can be developed into small molecule sensors or CNT based catalysis and theoretical studies are important methods to predict their structures and properties. The Project I of this thesis focused on the development of a model that is specifically used for the study of transition metal doped single wall CNT (SWCNT) systems at a reduced computational cost for neutrally charged systems. Longer SWCNT segments of about 20 layers of carbon and terminated by hydrogen or nitrogen atoms with single transition metals doped in exo and endo modes as well as the transition state of the exo-endo transformation were utilized to benchmark our new model. Numerical results show that the new model with a shorter length of SWCNT segment is able to accurately represent geometrical and electronic structures of longer ones based on conventional termination schemes. Computational results further indicate that the transition barrier heights between the exo and endo modes of various metal-doped systems are ranging from 0.1 to 5 eV. Further research work include the study of other elements or metal clusters as dopants, the ionized version of our doped SWCNT systems, as well as to test with other functional methods. Finding the weakest bond within a molecule is crucial for understanding the mechanisms behind chemical reactions and the bond dissociation energy (BDE) is used frequently for locating the weakest bond. However, it requires one to perform tedious calculations of BDEs for all bonds within a molecule. To save time and computational resources, we made attempts to design reasonable yet simple structural indicators to identify weak chemical bonds in Project II. Based on the commonly available structural property indicators for bond strength, such as bond length, the Mulliken interatomic electron number, and the Wiberg bond order, we have created several new bond-strength indicators that can be directly used to efficiently identify almost all weak bonds. In the future, different chemical systems will be analyzed by the new indicators to test their reliabilities.